Improving fuel efficiency of automobiles has been an important task in recent years from the viewpoint of global environment protection. Accordingly, there has been a vigorous trend toward making vehicle body parts thin by increasing the strength of vehicle body material to reduce weight of vehicles. However, these vehicle body parts, each generally manufactured by press-forming a steel sheet having a desired strength, exhibit deteriorated formability as strength thereof increases and cannot be reliably formed into a desired member shape.
In view of this, GBP 1490535 discloses what is called “hot/warm press forming” as a method for manufacturing a member by press-forming a heated steel sheet in a mold and then immediately and rapidly cooling the steel sheet to increase the strength thereof. The method has already been applied to manufacturing some members requiring TS in the range of 980 MPa to 1470 MPa. This method characteristically alleviates the aforementioned formability deterioration problem as compared to what is called “cold press-forming” at room temperature, and can highly increase the strength of a subject member by utilizing a low-temperature transformed microstructure obtained by water-quenching.
However, some structural members for use in automobiles, e.g. a side member, require high ductility in terms of ensuring safety during a collision and the conventional hot/warm press-formed member as disclosed in GBP 1490535 does not necessarily exhibit satisfactory ductility in this regard.
In view of this, there has been proposed as disclosed in JP-A 2007-016296 a hot press-formed member manufactured by hot press-forming a steel sheet at a temperature in the two-phase region of (ferrite+austenite) such that the steel sheet has: dual-phase microstructure constituted of 40%-90% ferrite and 10%-60% martensite by area ratio after hot press-forming; TS in the range of 780 MPa to 1180 MPa class; and excellent ductility of total elongation in the range of 10% to 20%.
However, the hot press-formed member disclosed in JP-A 2007-016296 does not reliably exhibit sufficient ductility, although the member has tensile strength around 1270 MPa. Therefore, it is still necessary to develop a member having high strength and excellent ductility in a compatible manner to achieve further reduction of automobile body weight.
It could therefore be helpful to provide a high strength press-formed member having tensile strength of at least 980 MPa and excellent ductility of (TS×T. EL.)≧17000 (MPa·%), as well as an advantageous manufacturing method of the high strength press-formed member.